Question

I was trying to do this,

class Outer {
public:
    struct inner_t {
        void foo()
        {
            printf("%d", _x);
        }
    };
    int _x;
};

int main()
{
    Outer o;
    o._x = 10;
}

The above can’t compile with errors:

error: invalid use of non-static data member ‘Outer::_x’

But according to this post, inner_t can indeed access Outer::_x, what’s wrong?

Answer 1

The problem is: inner_t does not know an instance of Outer to read _x from.

If you had written (for example):

void foo(const Outer *o)
{
    printf("%d", o->_x);
}

Or if _x was a static member of Outer.
Then it should work (at least it will give no error).